1. Field of the Invention
The present invention relates to an image forming apparatus of a digital type, and more particularly, it relates to an image forming apparatus having a table for coordinating, or corresponding, an image signal from a reading means to a recording means.
2. Related Background Art
In some known image forming apparatus of a digital type, a conversion table is provided for coordinating an image signal from a reading means for reading an original to a recording means. However, due to the deterioration and/or contamination of parts in the apparatus and change in environmental conditions, such conversion table cannot perform the proper conversion, thus creating a problem that an output image differs from a desired one.
As an example, in some image forming apparatus of a digital type, an image signal conversion table (referred to as a "LUT" or "look-up table" hereinafter) is used for correcting the output property of the apparatus and for emphasizing a specific density area. In such image forming apparatus, generally, an image reading means, an image processing means, an image writing means and the like are provided, and the look-up table is incorporated into the image processing means and serves to convert an input image signal input from the image reading means to the image processing means into an output image signal which is in turn input to the image writing means.
A curve (a) shown in a graph of FIG. 2 indicates a relation between an output image signal S2 and an output image density D1 in an electrophotographic printer. Even when the input image signal sent from the image reading means (reader) reflects image density D0 (not shown) of the original correctly, if the data is output as it is, the output image density D1 will not be proportional to the output image signal S2, as apparent is from the output property shown by the curve (a) in FIG. 2, thus deviating from the original image density D0. In order to eliminate the difference between input image density and the output image density, it is known that the image signal must be converted by the look-up table.
A method for forming the look-up table for corresponding the input image density to the output image density will be explained with reference to FIG. 2, if it is assumed that the input image signal S1 (not shown) from the image reading means reflects the read density, by making the relation between the output image signal S2 and the output image density D1 linear (as shown by a straight line (b) in FIG. 2), it is possible to coincide the input image density with the output image density (incidentally, there is density which cannot be reproduced, in view of the property of the printer). If the maximum reproduceable density of the printer is 1.5 and the output image signal is in a range from 00h to FFh (hexadecimal numbers), as shown in FIG. 2, the output image density D1 corresponding to the output image signal S2 of 80h is desirable to have a value of 0.75 from the relation as shown by the straight line (b). However, actually, since the printer has the property as shown by the curve (a), the output image density D1 of 0.75 is attained by the output image signal S2 of 57h. Accordingly, one data in the look-up table must serve to convert the output image signal S2 of 80h into an output image signal of 57h. In this way, the look-up table serves to convert the output image signals from 00h to FFh into values for correcting the respective output properties. Incidentally, a point Q where the curve (a) and the straight line (b) are crossed in FIG. 2, since the density on the curve are the same as the density on the straight line, the output image signal (48h) in this case is not required to be converted.
However, since the look-up table is formed in consideration of the output property of the printer regarding the image density as mentioned above, if the output property of the printer is varied, for example, by the "deterioration and/or contamination" of an image forming means of the printer, the look-up table cannot perform the proper correction. Further, if the value to be corrected is too great (for example, when the actual output property greatly deviates from the ideal output property (straight line) it should be corrected to a value on the straight line), the look-up table has the tendency to emphasize the minor change in the output property greatly, and thus, the look-up table is not suitable in the practical use.
Further, also in an image forming apparatus having a light amount table for coordinating an image density signal from a reading means to a laser amount of a recording means, when parts of the apparatus are deteriorated or the environmental conditions are changed, the light amount table cannot operate correctly, thus creating a problem that a proper image cannot be reproduced.